Air mattresses are used with cots and beds to provide yieldable body supports. The air mattresses are air-filled bags having flexible air impervious sheet members secured together to form one or more longitudinal chambers for accommodating air under pressure. Air mattresses have air inlet tubes that are used with hand-operated pumps, bag pumps, and other sources of air under pressure to inflate the mattresses. Plugs are used to close the tubes after the mattresses are inflated with air. Conventional air mattresses do not have pressure relief valves so that they can be easily over-inflated, causing rupture of the mattress material or the separation of the seams. An over-inflated air mattress is relatively hard and can become mis-shaped and wobbly.